(a) Field of the Invention
The present invention relates to spherical carbon, and particularly to spherical carbon with a size of several to several tens of micrometer that can be used for anode active material for a lithium secondary battery. The present invention also relates to a method for preparing the spherical carbon.
(b) Description of the Related Art
As anode active material for a lithium secondary battery, graphite material such as natural graphite and artificial graphite, non-graphitizable carbon or hard carbon, graphitizable carbon or soft carbon, etc. are used. Graphitized g-MCMB (Graphitized MesoCarbon MicroBeads, product of Japan Osaka Gas Chemical Co.), which is one type of artificial graphite, is used the most.
The reasons why g-MCMB is preferred are that a battery using the material has a high energy density per battery volume because the size of carbon particles is several to several tens of micrometers and thus g-MCMB can achieve a high rate of packing in a battery, and a battery employing g-MCMB has a small initial irreversible capacity because the carbon particles are spherical such that they have a small specific surface area. Initial irreversible capacity refers to the charge required for forming a passivation film while electrolyte is decomposed on a surface carbon when initially charging a lithium secondary battery, and the required packing is a factor in limiting battery capacity because it cannot be used in a subsequent discharge process. Such film forming is an unavoidable process when using carbon material as an anode. It is therefore important to minimize film forming, which is possible by minimizing the specific surface area of carbon that is used as an anode material.
Non-graphitizable carbon is produced by carbonizing a resin precursor at 700 to 1500° C. under inert atmosphere. Hence, non-graphitizable carbon has a low unit cost of production compared to artificial graphite requiring a high temperature heat treatment of 2500° C. or more. Further, non-graphitizable carbon has a reversible capacity of 400 mAh/g or more compared to graphite carbon (natural carbon, artificial carbon such as g-MCMB), which has a reversible capacity of approximately 300 mAh/g (the theoretical reversible capacity is 372 mAh/g).
Practically, there are two reasons why non-graphitizable cannot be widely used for a battery.
First, the crystallinity of non-graphitizable carbon is not high and non-graphitizable carbon includes fine pores and thus has a low density, while graphite carbon has a crystalline structure with a high crystallinity and well-developed graphite layers and thus it has a high density. Since the non-graphitizable carbon has a low density, the volume of an anode becomes large if it is packed in a battery such that the energy density per battery volume becomes low. Specifically, assuming that carbon of the same weight is packed, non-graphitizable carbon occupies more volume than graphite carbon.
Second, general non-graphitizable carbon must undergo a pulverizing process in order to be used for a battery because it is produced in a massive form. Particles of pulverized carbon have an irregular shape and a large specific surface area. A packing density becomes low because the shapes of the particles are irregular, and, because of the large specific surface area, an initial irreversible capacity becomes large such that an initial coulomb efficiency is lowered.
Accordingly, in order to take advantage of the inexpensive cost and high reversible capacity properties of non-graphitizable carbon, a method for making particles of the carbon spherical requires examination. Specifically, if the particles of non-graphitizable carbon are spherical, a tap density will be high, thereby allowing a large amount of the carbon to be packed. Also, the specific surface area of the carbon will be small and thus an initial irreversible capacity can decrease. If non-graphitizable carbon can be produced in spherical form, the problems of non-spherical non-graphitizable carbon, i.e., a low packing density and large initial irreversible capacity, can be simultaneously solved.
Graphitizable carbon refers to carbon that becomes artificial carbon when heat-treated at a high temperature of 2500° C. or more after carbonizing a pitch precursor at 700 to 1500° C. under inert atmosphere. As spherical artificial carbon, G-MCMB is widely used, which is prepared by heat-treating pitch at 300 to 500° C. to make mesophase spherulite, then by performing the processes of cooling, extracting with solvent, carbonizing and graphitizing. However, this process has a low yield and the production cost is high.